The long term objective of this proposal is to improve the therapy of lower-grade astrocytoma (LGA) by glioma by defining how expression of the mutant IDH1 protein contributes to alternative lengthening of telomeres (ALT). LGA account for 20% of all malignant glioma, and nearly all progress over time to fatal high-grade glioma. Most LGA express a mutant form of isocitrate dehydrogenase (IDH1mut) that generates the oncometabolite 2-hydroxyglutarate, alters gene expression, and drives tumorigenesis. An unexplored aspect of how IDH1mut drives gliomagenesis is its potential link to telomere regulation. Telomeres are DNA repeats at the ends of chromosomes that, in the absence of TERT, shorten with each cell cycle, eventually leading to dissociation of a telomere-protective sheltrin cap, chromosomal fusion, and cell death. Telomeric dysfunction is resolved in most glioma cells by TERT reactivation. Virtually all LGA, however, use an alternative, homologous recombination (HR)-based mechanism to elongate telomeres and survive in the absence of TERT. We recently showed that expression of IDH1mut in an ATRX-deficient background was sufficient to drive the ALT phenotype in p53/pRb- deficient human astrocytes. These ALT cells, as well as IDH1mut LGA, consistently downregulated RAP1 and XRCC1, and re-expression of XRCC1 and/or RAP1 suppressed the ALT phenotype. RAP1 is part of the sheltrin complex, and its loss can cause telomere uncapping. XRCC1 in turn is a critical component of the alternative non-homologous end joining (aNHEJ) pathway that generates lethal chromosome end-to-end fusions following telomere uncapping. Based on these observations we hypothesize that IDH1mut-driven down- regulation of RAP1 and XRCC1 leads to telomere dysfunction and inhibition of the aNHEJ pathway, enabling IDH1mut /ATRX-deficient cells to use HR and ALT to resolve telomeric dysfunction and escape cell death. This hypothesis will be tested by determining 1) if downregulation of RAP1 causes telomeric dysfunction, and if this contributes to IDH1mut- driven ALT, 2) if downregulation of XRCC1 changes the pathway by which uncapped telomeres are repaired, and if this contributes to IDH1mut- driven ALT, and 3) if IDH1mut-driven changes in DNA repair provide collateral therapeutic vulnerability.